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modbus_new_rtu

Name

modbus_new_rtu - create a libmodbus context for RTU

Synopsis

modbus_t *modbus_new_rtu(const char *device, int baud, char parity, int data_bit, int stop_bit);

Description

The modbus_new_rtu() function shall allocate and initialize a modbus_t structure to communicate in RTU mode on a serial line.

The device argument specifies the name of the serial port handled by the OS, eg. "/dev/ttyS0" or "/dev/ttyUSB0". On Windows, it's necessary to prepend COM name with "\.\" for COM number greater than 9, eg. "\\.\COM10". See http://msdn.microsoft.com/en-us/library/aa365247(v=vs.85).aspx for details

The baud argument specifies the baud rate of the communication, eg. 9600, 19200, 57600, 115200, etc.

The parity argument can have one of the following values:

  • N for none
  • E for even
  • O for odd

The data_bits argument specifies the number of bits of data, the allowed values are 5, 6, 7 and 8.

The stop_bits argument specifies the bits of stop, the allowed values are 1 and 2.

Once the modbus_t structure is initialized, you must set the slave of your device with modbus_set_slave and connect to the serial bus with modbus_connect.

Return value

The function shall return a pointer to a modbus_t structure if successful. Otherwise it shall return NULL and set errno to one of the values defined below.

Errors

  • EINVAL, an invalid argument was given.
  • ENOMEM, out of memory. Possibly, the application hits its memory limit and/or whole system is running out of memory.

Example

modbus_t *ctx;

ctx = modbus_new_rtu("/dev/ttyUSB0", 115200, 'N', 8, 1);
if (ctx == NULL) {
    fprintf(stderr, "Unable to create the libmodbus context\n");
    return -1;
}

modbus_set_slave(ctx, YOUR_DEVICE_ID);

if (modbus_connect(ctx) == -1) {
    fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
    modbus_free(ctx);
    return -1;
}

See also